Question: Hissing Normal? New Power Brake Booster

[fine69]

I replaced my power brake booster after getting home from work last night. It's a genuine GM restoration part, and a tag on it indicated that it had been leak-tested prior to shipping.

I completed it around 9:00 - busted knuckles and all. After installing it, I noticed a somewhat faint hissing sound when the engine is running. However, the hissing stops when depressing the brake pedal. When I push really hard on the pedal and hold it, the RPMs drop slightly. This is opposite the problem I had prior to replacing the power booster - before, the hissing sound would begin when I depressed the brake pedal.

Not sure, but I think the sound is emanating from the booster's check valve - I noted that the valve looks like it comes with a small pin hole on the backside - it was dark, so I'll need to look closely today.

I have to take the car for a test ride, but the brakes seemed to be functioning normal (going up and down the driveway last night).

Is this hissing normal (perhaps I just never noticed it before)?


:confused

 

[JohnZ]

No, it's not normal to hear the hissing sound all the time. The only time you should hear it (if at all) is when you push on the pedal and air rushes in the back of the booster to replace the vacuum behind the booster diaphragm; there's a filter in the back of the booster that's supposed to minimize the noise of the air rushing in.

Do you have some free play at the top of pedal travel? If not, you may be partially depressing the pushrod and opening the valve just enough to start it moving, which can also cause light application of the brakes.

 

[fine69]

No, it's not normal to hear the hissing sound all the time. The only time you should hear it (if at all) is when you push on the pedal and air rushes in the back of the booster to replace the vacuum behind the booster diaphragm; there's a filter in the back of the booster that's supposed to minimize the noise of the air rushing in.

Do you have some free play at the top of pedal travel? If not, you may be partially depressing the pushrod and opening the valve just enough to start it moving, which can also cause light application of the brakes.

Thanks John. I do not believe I have much free play at the top of the pedal travel. Is there a way to adjust this - if that's called for?

Ralph

 

[JohnZ]

Yup - the clevis (which attaches the booster pushrod to the brake pedal) is threaded on the pushrod; loosen the jam nut, turn the pushrod so you have 1/8" to 1/4" of pedal free play at the top of pedal travel, and tighten the jam nut.

 

[White75]

Yup - the clevis (which attaches the booster pushrod to the brake pedal) is threaded on the pushrod; loosen the jam nut, turn the pushrod so you have 1/8" to 1/4" of pedal free play at the top of pedal travel, and tighten the jam nut.

+1 on this. I had a similar issue and adjusting the rod took car of it. Replacing a PB is a PITA and I don't ever want to have to do that again

 

[fine69]

Thanks Guys... yes, it was a pain to replace. I believe everything is a-okay now... thanks again.

Ralph